1. Field of the Invention
The present invention relates to LC filters, and more particularly, relates to a stacked LC filter for use at high frequencies.
2. Description of the Related Art
FIG. 8 shows a three or more stage band-pass filter allowing a signal in a particular frequency band to pass through it. The band-pass filter is a three-stage band-pass filter having a first stage resonator Q1, a second stage resonator Q2 and a third stage resonator Q3. These LC resonators Q1 to Q3 are electrically coupled via coupling capacitors Cs1 and Cs2 between adjacent resonators.
FIGS. 9 to 11 show a stacked band-pass filter having the above-described known circuit construction. An LC filter 1 having the first stage to the third stage LC resonators Q1 to Q3 are disposed in a layered body 21 constructed by stacking quadrangular ceramic sheets 2. The LC filter shown in FIGS. 9 to 11 is equivalent to the circuit shown in FIG. 8.
Inductors L1 to L3 of the LC resonators Q1 to Q3 include inductor patterns 3, 4, and 5, respectively. Capacitors C1 to C3 of the LC resonators Q1 to Q3 include capacitor patterns 9, 10, and 11, and edge portions 6, 7, and 8 of the inductor patterns 3, 4, and 5 which face the capacitor patterns 9, 10, and 11, respectively. The above LC resonators Q1 to Q3 are electrically coupled via the coupling capacitors Cs1 and Cs2 which include the capacitor patterns 9 to 11 and coupling capacitor patterns 12 and 13 which face these capacitor patterns 9 to 11. These LC resonators Q1 and Q3 are capacitively-coupled with an input capacitor pattern 14 and an output capacitor pattern 15, respectively. Shielding patterns 16a and 16b are provided such that the inductor patterns 3 to 5 and capacitor patterns 9 to 15 are held therebetween.
The layered body 21 has an input terminal electrode 26, an output terminal electrode 27, and shielding electrodes 28 and 29 disposed as shown in FIG. 10. The input terminal electrode 26 is connected to the input capacitor pattern 14, and the output terminal electrode 27 is connected to the output capacitor pattern 15. The shielding terminal electrode 28 is connected to the lead portions of the inductor patterns 3, 4, and 5 as well as to edge portions of shielding patterns 16a and 16b. The shielding terminal electrode 29 is connected to the lead portions of the capacitor patterns 9 to 11 as well as the other edge portions of the shielding patterns 16a and 16b. 
In the conventional LC filter 1, positions of poles (singular points) in attenuation characteristics cannot be easily adjusted. To change the positions of the poles, the shapes of the inductor patterns 3 to 5, the shapes of the capacitor patterns 9 to 11, and the shapes of the coupling capacitor patterns 12 and 13 must be adjusted. Therefore, almost all patterns have to be redesigned.
In order to overcome the problems described above, preferred embodiments of the present invention provide a stacked LC filter in which the positions of poles in attenuation characteristics can be easily adjusted
A stacked LC filter of a preferred embodiment of the present invention includes a layered body including a plurality of insulating layers, a plurality of inductor patterns, and a plurality of capacitor patterns stacked on each other, at least three LC resonators disposed in the layered body having a plurality of inductors defined by the inductor patterns, and a plurality of capacitors defined by the capacitor patterns which face the inductor patterns, at least two coupling capacitor patterns, stacked in the layered body arranged to coupled the LC resonators to one another, and a pole adjusting pattern stacked in the layered body, facing the at least two coupling capacitor patterns.
The arrangement of the pole adjusting pattern facing at least two coupling capacitor patterns allows the positions of poles in attenuation characteristics to be easily adjusted. For example, when the area of the overlapping portion between the coupling capacitor patterns and the pole adjusting pattern is increased, an electrostatic capacitance generated between the pole adjusting pattern and the coupling capacitor patterns is increased, which thereby increases the spacing between poles. Conversely, when the area of the overlapping portion is decreased, the spacing between poles is decreased. Therefore, the stacked LC filter of preferred embodiments of the present invention has superior attenuation characteristics, and consequently a superior duplexer can be produced using this stacked LC filter.
In the stacked LC filter, each inductor of each LC resonator has a multi-layered structure defined by at least two stacked inductor patterns having a substantially identical shape, with the insulating layers provided therebetween.
The multi-layered inductor arrangement prevents increased magnetic fields from being generated at the edge portions of inductor patterns. As a result, the Q-factor is greatly improved.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the drawings attached hereto.